Addition of HBr to Alkenes
Description: Treatment of alkenes with hydrobromic acid will result in the formation of alkyl bromides.
Notes: This is an addition reaction. Note that the bromine always ends up at the more substituted carbon of the alkene (Markovnikoff-selectivity)
Notes: When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. Note the third example – where Markovnikoff’s rule gives no clear preference, a mixture will be obtained. Note that the right hand molecule of the third example contains a stereocenter, which will be obtained as a racemic mixture (1:1 mixture of enantiomers).
Mechanism: Electrons from the C1-C2 π bond attack the hydrogen of HBr, expelling the bromide anion and leading to the formation of a carbocation (Step 1, arrows A and B). Note here that the carbocation preferentially forms on C2 (secondary) and not C1 (primary) since secondary carbocations are more stable. The bromide anion then attacks the carbocation, leading to formation of the alkyl bromide (Step 2, arrow C)
Notes: Since the carbocation is planar (flat) there is no preferred direction of attack of the bromide ion. If it’s possible to form a stereocenter, a mixture of stereoisomers will be obtained.
Additional example (advanced) : If the molecule contains adjacent stereocenters, the two directions of attack on the carbocation will no longer be of equal energy and a mixture of diastereomers will be obtained. In this example, attack of the bromide ion on the less hindered “top face” of the carbocation is favored, and a mixture of products (diastereomers) will be obtained.
(Advanced) References And Further Reading:
- Early example
The Stereochemistry of the Addition of Hydrogen Bromide to 1,2-Dimethylcyclohexene
George S. Hammond and Thomas D. Nevitt
Journal of the American Chemical Society 1954 76 (16), 4121-4123
Early paper from the 50’s by Prof. George Hammond (of Hammond’s Postulate) on the mechanism of HBr addition to 1,2-dimethylcyclohexane. He prefers a concerted pathway, although that might due to the conditions he employs – in pentane, a very nonpolar solvent, polar intermediates are disfavored.
- Mechanistic studies
Hydrochlorination of cyclohexene in acetic acid. Kinetic and product studies
Robert C. Fahey, Michael W. Monahan, and C. Allen McPherson
Journal of the American Chemical Society 1970 92 (9), 2810-2815
Detailed kinetic studies of the addition of HCl to cyclohexene in acetic acid, discussing a possible third-order mechanism (rate = k[cyclohexene][HX]2).
- Experimental Procedure
SPIROANNELATION OF ENOL SILANES: 2-OXO-5-METHOXYSPlRO[5.4]DECANE
Lee, T. V.; Porter, J. R. Org. Synth. 1995, 72, 189
The first reaction in the above procedure involves two steps – addition of HBr across the double bond and converting the aldehyde to a dimethyl acetal.